Tropical Cyclogenesis Composites in the NCEP GFS Model

The National Hurricane Center (NHC) routinely forecasts the potential for tropical cyclone (TC) formation at lead times of 48 and 120 h. These forecasts are issued every 6 h during hurricane season and are expressed in terms of probabilities. The genesis forecasts are heavily dependent on numerical model guidance, particularly beyond 24 to 48 hours. Since what constitutes TC genesis in global model output is not obvious, operational forecasters are left to subjectively determine the likelihood of tropical cyclone formation from an examination of various model fields, such as mean sea-level pressure and 850-hPa relative vorticity.

We will present composites of analyses from the NCEP GFS model at the time of TC formation in the NHC best track over several years to attempt to quantify thresholds in the model fields that can be used to identify TC genesis. Ideally, these composites could be expanded to additional global models and yield a uniform set of standards from which forecasters could assess the prospects for TC genesis.